Ionisation chamber



IONISATION CHAMBER Filed June 16, 1958 FIGJ INVENTOR NICOLAAS WARMOLTZPAULUS PHILIPPUS MARIA SCHAMPERS United States Patent Fice i "3,030,538IONISATION CHAMBER Nicolaas Warmo'ltz-and Paulus Philippus Maria',Schampers, Eindhoven, Netherlands,^assignors' to North American PhilipsCompany, inc., New York, N.Y., a corporation ofDelaware Filed June 16,1958, Ser. No. 742,241 Claims priority, application Netherlands .lune28, 1957 5 Claims. (Cl. 313-93) This invention relates to ionisationchambers, especially for measuring gamma and X-rays.

For measuring 'y-rays and`X-rays, use is made ofthe ionisation vproducedby 'these rays in acertain volume of a gas `or gas mixture. "The gasvolume is enclosed'in a chamber the walls of which are made entirely orin part of vinsulating material ari'd'inl which two electrodes areprovided. Generally one electrode is applied to the wall while the.other is arrangedin the chamber. Each-electrode is connected to aconductor passing through the wall so that an electric potential is setup across them. The electrodes must 'be highly insulated from oneanother.

The wall material of ionisation chambers preferably consists of asubstance which with respect to its permeability to the ionisingradiation has about the same properties as air. Otherwise thesensitivity of the ionisation chamber will depend to a higher or lesserdegree upon the hardness of the rays. Particularly in'small ionisationchambers of small volume the measuring results can be materiallyiniluenced by the composition of the wall material. As the wallmaterial, use is frequently made of an organic substance which hassubstantially the same absorption and diffusion for the rays to bemeasured as air. In general, such materials are referred to as airwallequivalent materials.

ionisation chambers are increasingly used in radiation counters fordetecting ionising radiation and for safeguarding persons whose workentails that they can be exposed to radiation. Consequently theseapparatus must be permanently disposed in the vicinity of the person tobe protected and therefore their size is made s0 that they can readilybe worn on or in the clothing. Consequently the ionisation chambers usedmust be very small. Hence the electrodes are closely adjacent and theinsulation distance is slight, so that the Wall material must have avery high insulation resistance.

It is known to use organic substances, such as polystyrene, as the wallmaterial. The insulation resistance is suliciently high, but they have alimitation which restricts their use to ionisation chambers lled withair under atmospheric pressure. The material is not cornpletely airtightand consequently not completely reliable for chambers which are iilledwith a gas or gas mixture other than air or in which the pressure of thegas lling differs from the atmospheric pressure. In course of time thecomposition of the gas or the pressure in the ionisation chamber isslightly changed. In addition, it may be desirable for chambers whichare closed hermetically to be degassed in order to remove entirelydetrimental constituents, more particularly hydrogen. The organicsubstances cannot withstand the temperature required in this process.

It is an object of the present invention to avoid these disadvantages.It relates to an ionisation chamber the wall of which is airtight, hasvery high insulation resistance, can be heated at a suflciently hightemperature for the chamber to be completely degassed, while theradiation absorption is fairly similar to that of air, i.e., it is asubstantially air-wall equivalent material. According to the invention,the wall of an ionisation chamber is made 3 ,030,538 Patented 'Apr'. I7,"1 962 of Aa vitreous substance, composed of pure bo'ric 'acid anhydridewhichis .usually referred 'to Yas :boric oxide.

Glass `compositions areknown which contain 1a high percentageof'borieacid. *"Howeven'jthese -glasses do not possess the propertyrequired in manufacturing ionisation chambers, that the radiationabsorption inthe material is similar to that of air. The absorption 'isgreater, frequently even materiallyjgreaten l YOwing'to 'the'slightelectrical conductivityof the vitreous B203 the insulation distances'required'between the electrodes and the lead-in conductors'canb'esmall'and the chamber can have the small size required'tforitsuse as the'envelope of a radiation 'counter tolbe worn onor in theclothing.

An advantage of the substance consists lin .that itcan be sealed ltoaluminum. 'Conductors of this material can be sealed in the walldirectly. Conductorsofcopper are locally coatedwith a soft enamel 'as anintermediate layer.

As is known, the vitreous substance consisting of boric oxide is highlyhygroscopic. Consequently,\when it is used in an ionisation chamber inaccordance with'the invention, the wall must .be protected againstattack by moisture. lThe chamber canbe arranged inan'envelope filledwith dry air or in a'vacuum. 'I'he space surrounding theionisation'chamber Vwithin the envelope canibe used to accommodatethe'electr'ostatic indicating system which'is employed in radiationcounters for thedetection of the ionisingradiatio'n. -lf the ionisationchamber is to be used independently, a protective layer can be provided.This layer can consist of polyester or celluloid. After mounting, theionisation chamber can be immersed in a solution of one of thesesubstances and subsequently dried. If required, the immersion can berepeated until the layer has attained the required thickness.

In order that the invention may readily be carried out, reference ismade to the accompanying drawing, in which- FIG. 1 is a cross-sectionalview of an ionisation chamber in accordance with the invention, providedwith a protective coating,

FIG. 2 shows the ionisation chamber arranged in a protective envelope.

Referring now to the gures, the gas volume to be ionised is enclosed ina tubular chamber 1, the Wall of which consists of a vitreous substancemade of boric oxide. The gas volume may consist of air or a gas fillingmay be provided which absorbs X-rays to the same extent as air. Asuitable pressure of the gas lies between l and 11/2 atm. At both endsthe tube 1 is closed and provided with feed-through conductors 2 and 3made, for example, of aluminum or copper. The copper wires are sealed inthe wall 1 With the interposition of a vitreous enamel 4 and 5. An innerelectrode 6, which may consist of a graphite rod, is connected to thelead-in 2. The outer electrode 7 comprises a conductive substance,preferably carbon, which -is applied to the wall 1. Prior to filling thechamber with the ionisable gas, to remove detrimental constituents, suchas hydrogen, the chamber may be heated to a temperature of to 150 C.

On the Wall 1 a coating 8 of moisture-proof insulating material, made offor example polyester or celluloid, is applied In FIG. 2 the ionisationchamber 1 is situated Within a cylindrical or tubular housing 9 which isformed of an electrically conducting material that is pervious toradiation, such as for example aluminum. Within housing 9 is situated alens 10 forming one end wall of a small compartment 11 the other end ofwhich is closed by an insulating wall 12. One electrode 3 of theionisation chamber 1 extends through the wall 12 and the second 3electrode 2 supports an electroscope assembly 13, which is well known inthe art so that it is not described in de@ tail. The lens and the wall12 are spaced apart by a hollow cylindrical shell 14 havingapproximately the same Vouter diameter as the inner diameter of housing9. The space 11 is preferably lledA with dry air. Y

The assembly as far as it is shown in FIG. 2 may form part of aradiation detection device as described in U.S. patent specification2,613,327 to replace the electroscope assembly used therein.

What is claimed is:

1. An ionization chamber for 'measuring gamma rays, X-rays, and thelike, comprising a sealed envelope having a wall portion of substantialsurface area constituted of a substantial air-wall equivalent materialand separating the envelope interior from the outside environment, anionizable medium and electrode system Within the envelope, externalterminals sealedpin the envelope and connected internally to theelectrode system, said separating'wall portion Vconsisting ofelectrically-insulating, gas-tight, vitreous boric acid anhydride, andmeans surrounding the said ,Wall portion of boric acid anhydride andpreventing moisture from coming into contact therewith.

2. An ionization chamber as set vforth in claim 1, wherein thelast-named means comprises a moisture-impermeable coating provided onthe said Wall portion.

3. A chamber as set forth in claim 2 wherein the moisture-impermeablecoating is selected from the group consisting of a polyester andCelluloid.

4. An'ionization chamber for measuring gamma rays, X-rays, and the like,comprising a sealed envelope wholly constituted of a substantialair-wall equivalent material and separating the envelope interior fromthe outside environment, an ionizable medium and electrode system withinthe envelope, external terminals sealed in and through the envelope walland connected internally to the electrode system, said envelopeconsisting of electricallyinsulating, gastight, pure vitreous boricoxide B203, and means surrounding the said envelope and preventingmoisture from coming into contact therewith.

5. An ionization chamber as set forth in claim 4 wherein the last-namedmeans comprises a hollow enclosure containing a moisture-freeatmosphere.

References Cited in the le of this patent UNITED `STATES PATENTS2,238,777 Lemmers et al. Apr. l5, 1941 2,367,595 Marden Ian. 16, 19452,500,941 Friedman Mar. 21, 1950 2,536,991 Wollan Ian. 2, 1951 2,568,459Noel Sept. 18, 1951 2,573,999 Victoreen Nov. 6, 1951 2,596,080 Raper May6, 1952 2,639,389 Landsverk May 19, 1953 2,683,234 Lynch July 6, 19542,837,678 Hendee et al June 3, 1958 FOREIGN PATENTS 606,013 GreatBritain Aug. 4, 1948 OTHER REFERENCES Hanson et al.: Neutron DetectorUniform Sensitivity from 10 kev. to 3 mev., Physical Review, vol. 72,No. 8. October 15, 1947, pages 673-677.

1. AN IONIZATION CHAMBER FOR MEASURING GAMMA RAYS, X-RAYS, AND THE LIKE,COMPRISING A SEALED ENVELOPE HAVING A WALL PORTION OF SUBSTANTIALSURFACE AREA CONSTITUED OF A SUBSTATIAL AIR-WALL EQUIVALENT MATERIAL ANDSEPARATING THE ENVELOPE INTERIOR FROM THE OUTSIDE ENVIRONMENT, ANIONIZABLE MEDIUM AND ELECTRODE SYSTEM WITHIN THE ENVELOPE, EXTERNALTERMINALS SEALED IN THE ENVELOPE AND CONNECTED INTERNALLY TO THEELECTRODE SYSTEM, SAID SEPARATING WALL PORTION CONSISTING OFELECTRICALLY-INSULATING, GAS-TIGHT, VITREOUS BORIC ACID ANHYDRIDE, ANDMEANS SURROUNDING THE SAID WALL PORTION OF BORIC ACID ANHYDRIDE ANDPREVENTING MOISTURE FROM COMING INTO CONTACT THEREWITH.